Molecular Mechanism of Wnt/Planar Polarity Signaling Summary Directed cellular polarization as a key feature of organismal development is required for tissue and organ function and homeostasis. Planar Cell Polarity (PCP) is emerging as a fundamental mechanism regulating various morphogenetic processes including cartilage elongation in the limb, anterior-posterior (A-P) body axis elongation, neural tube closure, body hair orientation, orientation of inner ear sensory hair cells, left-right asymmetry and axon guidance in vertebrates. Mutations in PCP signaling components have been identified in human diseases such as brachydactyly type B1, Robinow syndrome, scoliosis, spinal bifida and epilepsy. Despite the fundamentally important roles of PCP, the mechanisms of PCP establishment by global instructive cues such as Wnts remain poorly understood and represent an exciting frontier in developmental and cell biology. The Wnt/Planar Cell Polarity (PCP) pathway is evolutionarily conserved and provides essential directional information during morphogenesis to orient cytoskeleton, cell division, cell migration, differential adhesion across cells, and to position cell extensions, such as cilia and axons. However, unlike the extensively studied Wnt/?-catenin pathway, Wnt signal transduction in the PCP pathway remains poorly understood. The PCP pathway is controlled by core PCP proteins including Van Gogh (Vang), Frizzled (Fzd) and Dishevelled (Dvl), which were originally identified in Drosophila. Wnt/PCP signaling in vertebrates is more complex and functionally diverse and vertebrate-specific features of PCP require rigorous genetic and biochemical studies of their own. The core PCP proteins are initially randomly distributed in the cell and gradually accumulate on one side of the cells instructed by global cues during PCP establishment. Wnt5a is a global cue required for establishing PCP in vertebrate long bone cartilage by inducing a novel receptor complex that contains Vang like 2 (Vangl2) and Ror2, a vertebrate specific PCP component. As a result, Vangl2 is phosphorylated in a Wnt5a dose-dependent manner and Vangl2 phosphorylation regulates its function. Our identification of Wnt induced PCP signalosome and Vangl2 phosphorylation as both an important readout and transducer of Wnt/PCP signaling opens a new door to find missing links in the Wnt/PCP signaling cascade. We propose to decipher novel Wnt5a signaling events that eventually lead to PCP establishment with rigorous genetic and biochemical approaches. In Specific Aim 1, we will define the functions and regulations of Vangl2 phosphorylation in vivo. In Specific Aim 2, we will define the molecular mechanism whereby Wnt5a signal is transduced through Vangl2 in the PCP pathway. In Specific Aim 3, we will identify additional regulatory components in Wnt/PCP signaling. Given the fundamental roles of Wnt/PCP signaling in many morphogenetic processes and identified WNT5A, VANGL and ROR2 mutations in human diseases, our studies of Wnt5a/PCP signaling in vertebrates will advance our understanding of Wnt signaling in human biology and pathology.